For seatbelt apparatuses to be mounted in vehicles such as cars, various seatbelt retractors including a pretensioner have been developed. In an initial stage of an emergency, such as a vehicle collision, when a deceleration higher than the normal deceleration is applied to the vehicle, this pretensioner rotates a spool of a seatbelt retractor in a seatbelt retracting direction by using reaction gas generated by a gas generator, thereby retracting a seatbelt around the spool. This quickly removes slack from the seatbelt and applies tension to the seatbelt so as to increase the occupant restraint force.
As an example of a pretensioner of the related art, there is known a pretensioner in which a plurality of force transmission members formed by balls are stored in a pipe. In case of an emergency, these force transmission members are moved along the interior of the pipe by receiving gas pressure of reaction gas, and press a plurality of pressed portions of a ring gear. A spool is rotated by the ring gear in a seatbelt retracting direction (for example, see PTL 1).
In the pretensioner described in PTL 1, the balls, whose rotating forces are substantially lost after the completion of the rotation of the ring gear, come out from a cutout portion of the pipe, separate from the ring gear, and are received in a special ball storage chamber provided in the pretensioner. When the ball storage chamber is thus provided, the size of the pretensioner is increased, and the size of the seatbelt retractor is inevitably increased.
Since the balls are stored in the ball storage chamber, the number of balls is relatively large, and the length of the expensive pipe is large. This increases the cost of the pretensioner of the related art. Moreover, since the length of the pipe needs to be long to a certain extent, a gas generator is forced to be provided on a lower side of the seatbelt retractor. This complicates a mounting operation of the gas generator. If the gas generator is forcibly provided on an upper side of the seatbelt retractor, routing of the pipe is difficult, and the size of the seatbelt retractor increases.
In contrast, there is known a pretensioner in which a plurality of pressed portions of a ring gear are sequentially pressed by a plurality balls serving as mass bodies to rotate the ring gear. The balls, whose rotating forces are substantially lost after the completion of the rotation of the ring gear, are received between adjacent pressed portions of the ring gear and are moved along with the rotation of the ring gear (for example, see PTL 2).
In the pretensioner described in PTL 2, since the balls are received in recesses provided between the adjacent pressed portions of the ring gear, there is no need to form a special ball storage chamber in the pretensioner. Therefore, this pretensioner does not have the above-described problem of the pretensioner described in PTL 1.